The present invention relates to an artifical halftone processing apparatus and, more particularly, to an artificial halftone processing apparatus of the type binarizing input image information on the basis of the densities of pixels and combining a pluality of sets of resultant binary data to produce multi-level tone information.
With a laser printer or similar recorder, only a predetermined recording density is available for all the pixels so that density is rendered in two levels only, i.e. black and white. It has been customary, therefore, to provide this kind of recorder with an implementation for rendering halftone artificially which relies on dither processing. Dither processing is such that the number of recording pixels and that of non-recording pixels are adjusted in each of multiple regions each consisting of a plurality of output pixels so as to increase or decrease the average tone of each region, thereby rendering halftone. Adjusting the number of recording pixels and that of non-recording pixels on an 8.times.8 matrix region basis, for example, is successful in rendering sixty-five consecutive tones. For this kind of processing, use is made of a threshold table in which a different threshold value is assigned to each pixel position of a matrix having a predetermined number of pixels. The tones of the individual pixels of input image information are compared one-to-one with the threshold values of the threshold table, so that output pixels are provided with either a recording level or a non-recording level on the basis of the result of comparison. Some laser printers or similar recorders have a hardward dither processing device capable of executing halftone processing at high speed in order to implement the dither processing.
A dilemmatic situation with artificial halftone processing described above is that increasing the size of the matrix for halftone processing allows delicate tones to be rendered, but it lowers the resolution of an image. Further, when a figure or similar image is equal to or greater in size than the matrix, critical errors occur in the representation of tones of an output figure resulting in the image quality being noticeably degraded. On the other hand, when the size of the matrix is reduced, the number of tones which can be renderd is reduced to in turn aggravate the tone errors of an output image, again resulting in poor image quality. The dither processing device installed in the prior art recorder has a dither matrix whose size and configuration are fixed and generates information representative of pixel positions in the matrix by using a counter which counts scan synchronizing signals synchronous to recording operations. Hence, when a figure constituting an image is relatively small, the tones of an output image are poor; when the matrix is small, tones cannot be rendered delicately with the result that the tone errors of an output image are aggravated. Furthermore, assume that image data undergone halftone processing are written in a memory instead of being delivered to a printer or similar output unit, and that the image data have been dither-processed with respect to two levels. Then, since addresses of a memory are usually assigned on an eight bit (one byte) basis, eight pixels of data have to be stored together.
The various problems discussed above will be eliminated if the halftone matrix is adjustable in size in matching relation to the size of a figure, i.e., if delicate tone representation is availble with relatively large figures and the tone representation errors and the decrease in resolution particular to relatively small figures are minimized. This kind of processing is complicated and cannot be executed unless all the halftone processing is implemented by software. In image processing, however, since the number pixels to be processed is huge, executing the whole halftone processing pixel by pixel by software would required an extremely long period time.